The automotive industry has had notable success in the reduction of regulated gaseous emissions from the use of hydrocarbon fuels in mass produced automobiles. For gasoline spark ignited engines, the gaseous emissions fall into two categories:
(1) evaporative emissions--which relate to unburned fuel vapors escaping from the vehicle's fuel tank, and PA1 (2) tailpipe emissions--which relate to emissions from the exhaust of the engine and include unburned and partially burned fuel, carbon monoxide, and oxides of nitrogen.
In the mid 1970s, catalytic converters and closed loop fuel control was adopted almost universally in the United States and progressively in other countries. As stricter emission control requirements were written into law, microprocessor-controlled fuel injection eventually became widespread, allowing for more elaborate and sophisticated control systems and fuel control strategies.
Early automotive control systems often used emulations of the mechanical controls that had been replaced by electronically-actuated devices. Simple physical and empirical strategies with primarily tabular calibrations were used in order to be compatible with the limited microprocessor capacity on-board the vehicle. Current state-of-the-art low emission systems utilize more advanced controls strategies that include mathematics and physics-based models of the complex chemical, thermodynamic, mechanical, and electrical processes that exist in the automobile. This modeling and control strategy is implemented using software which provides designers with a mix of advanced controls techniques and thrifty empiricism that they can use in providing efficient and effective engine control logic.
In state-of-the-art low emission gasoline vehicles, both evaporative emissions and tailpipe emissions have been reduced by more than 90% from previous uncontrolled levels. The reduction in evaporative emissions has been achieved largely by use of evaporative emission control systems that utilize a charcoal canister to store fuel vapors from the fuel tank, with periodic purging of the vapors into the air intake manifold of the engine where they are drawn into the engine cylinders and burned. However, the objective of further reducing the emissions to near zero levels gives rise to a conflict between the need for aggressive purging of the charcoal canister to control evaporative emissions and extremely precise control of engine Air/Fuel ratio for tailpipe emissions control. For example, the design of high pressure fuel injection systems has often included the use of high-flow re-circulation of fuel (pumped from the fuel tank to the engine and back to the tank). This would allow the fuel injectors to be maintained at lower operating temperatures, even in applications where underhood temperatures and fuel injector location would otheirwise have resulted in excessive fuel injector temperatures. This has helped avoid phenomena such as vapor lock and is also considered desirable for the longevity and precision of the fuel injector. However, this fuel control approach is at odds with the need to keep tank temperatures low to avoid evaporative running losses in extreme conditions. In addition, new requirements for On-Board Refueling Vapor Recovery (ORVR), On-board Diagnostic (OBD II and EOBD) and real-time and high temperature evaporative emission testing have created a strong need to more capable purge strategies.
One problem with currently available evaporative emission control systems is that they do not always prevent running losses; that is, loss of fuel vapors through the canister vent valve that is connected between the charcoal canister and the surrounding atmosphere. These running losses typically occur under conditions in which there is a large degree of fuel evaporation that cannot be purged at a high enough rate due to the current engine operating conditions. For example, in hot weather with the engine idling, the evaporation rate within the fuel tank may be greater than the current purge capacity of the engine, since it is running at idle. In both of these circumstances, pressure within the fuel tank due to the evaporating fuel, may actually force fuel vapors through the canister and out to the atmosphere through the canister vent valve. This problem is exacerbated by the use of high volatility fuels.
In response to the potentially high running losses that can occur with volatile gasoline blends in extreme temperature conditions, California has been the first to introduce legislation demanding reformulation requirements that include a mandate for special low volatility fuel. While this has served to drive the content of butane in California gasoline to lower levels, higher volatility fuel is still available in other states and countries. Moreover, as is known, the volatility of fuel is typically varied seasonally, with the higher volatility fuel being distributed and used during the winter months. Each spring there is normally a regionally applied cut-off date for the production and distribution of volatile winter grade fuel. Unseasonably warm weather or delays in selling and consuming this fuel can cause high volatility fuel to be present in vehicles operating in high temperature conditions. Also, the use of alcohol blends has been encouraged to achieve potentially lower tailpipe emissions. However, alcohol gasoline blends tend to be very volatile in extreme temperature conditions (even when the low temperature volatility is similar to normal commercial gasoline).
Similar issues exist in many hot climate countries around the world. Notable air quality improvements may be achieved in crowded urban markets by the development of emission control systems that can be more tolerant of volatile fuels in hot weather conditions. Improved control systems could reduce the need for fuel reformulation and enforcement of same and thus avoid costs to the oil industry and thus, indirectly, to the consumer.
One historical approach to address the problem of excessive vaporization of gasoline in the fuel tank has been the use of tank pressure control valves (TPCV). These were installed between the fuel tank and the charcoal canister in order to allow tank pressure to be above atmospheric temperature during potential evaporative emission conditions. While this hardware was relatively inexpensive and usefull in reducing evaporative emissions in some conditions, it had certain undesirable effects that has resulted in its used being discouraged by governmental regulators. Firstly, in refueling events a puff of evaporative emissions could result when the fuel tank temperature was hot and under some differential pressure. The so-called puff losses were lower than the potential evaporative emissions that might have otherwise occurred during an entire trip but the rapid loss of vapor in the presence of the vehicle operator during refueling was very undesirable. Secondly, in failure modes (where a pinhole leak existed in the fuel tank or associated hoses and connections) the TPCV prevented escaping vapors from being passed through the charcoal canister and thus resulted in dramatically increased evaporative emissions--similar to those of a vehicle without any evaporative emission control system. As a result of these problems, the use of TPCVs has largely been abandoned. Accordingly, there exists a continuing need for an evaporative emission control system that can reduce if not eliminate running losses.